Methylation-dependent DNA discrimination in natural transformation of <i>Campylobacter jejuni</i>

Abstract

<i>Campylobacter jejuni</i>, a leading cause of bacterial gastroenteritis, is naturally competent. Like many competent organisms, <i>C. jejuni</i> restricts the DNA that can be used for transformation to minimize undesirable changes in the chromosome. Although <i>C. jejuni</i> can be transformed by <i>C. jejuni</i>-derived DNA, it is poorly transformed by the same DNA propagated in <i>Escherichia coli</i> or produced with PCR. Our work indicates that methylation plays an important role in marking DNA for transformation. We have identified a highly conserved DNA methyltransferase, which we term <i>Campylobacter</i> transformation system methyltransferase (<i>ctsM</i>), which methylates an overrepresented 6-bp sequence in the chromosome. DNA derived from a <i>ctsM</i> mutant transforms <i>C. jejuni</i> significantly less well than DNA derived from <i>ctsM</i>^<i>+</i> (parental) cells. The <i>ctsM</i> mutation itself does not affect transformation efficiency when parental DNA is used, suggesting that CtsM is important for marking transforming DNA, but not for transformation itself. The mutant has no growth defect, arguing against ongoing restriction of its own DNA. We further show that <i>E. coli</i> plasmid and PCR-derived DNA can efficiently transform <i>C. jejuni</i> when only a subset of the CtsM sites are methylated in vitro. A single methylation event 1 kb upstream of the DNA involved in homologous recombination is sufficient to transform <i>C. jejuni</i>, whereas otherwise identical unmethylated DNA is not. Methylation influences DNA uptake, with a slight effect also seen on DNA binding. This mechanism of DNA discrimination in <i>C. jejuni</i> is distinct from the DNA discrimination described in other competent bacteria.